Flow throttling means reducing of a pressure either by a varying or a constant area throttling device, e.g. by a control valve or an orifice plate. When a gas flow is being throttled, part of the energy of the flow is converted into noise. The noise caused by flow throttling is dependent mainly on two factors, one of which is the flow velocity and the other is the pressure drop ratio across the throttling device. Pressure drop ratio here means the differential pressure divided by the inlet pressure, dp/p1, which is within the range 0 . . . 1, with the pressure decreasing in the flow direction.
The pressure drop ratio across the throttling device is dependent on the throttling area and on the flow velocity. With low pressure drop ratio values the flow is subsonic at the throttling point. Subsonic flow means that the flow velocity at the throttling is below sonic velocity. At the throttling point the flow velocity is increased due to changes in the flow cross-sectional area and gas density. In a subsonic flow the flow turbulence is the main source of noise. With high pressure drop ratios the flow is trans- or supersonic, which means that the flow velocity is close to the sonic speed or exceeds it. Shock waves forming in the flow, their vibration and interactions with the flow turbulence are then the main source of noise.
A shock wave is a very thin, almost discontinuous zone forming in a compressing gas flow and over which the gas density, pressure and flow velocity are changing significantly over a very short distance. In general terms, the noise caused by a trans- or supersonic flow is a much more serious problem than the noise caused by a subsonic flow. In other words, high pressure drop ratios across the throttling cause significantly more noise than low pressure drop ratios do.
Means of reducing the noise caused by gas throttling are either by reducing or suppressing the already existing noise or by preventing the generation of excessive noise levels. Existing noise can be reduced, for example, by insulation. The generation of high noise levels can be prevented by dividing the flow into many small jets, for example U.S. Pat. No. 4,108,210 (Luthe et al). The throttling can be divided into several steps, for example U.S. Pat. No. 4,921,014 (Tartaglia et al) or by designing the throttling geometry itself to be favourable in terms of noise, for example U.S. Pat. No. 4,149,563 (Seger). Dividing the flow into several small jets changes the frequency division of the noise and reduces the noise level sensed by the human ear. By dividing the throttling into several steps the pressure drop ratio existing across an individual step can be reduced and in this way the total noise level can be reduced. The degree of turbulence and the generation of shock waves can both be influenced by the throttling geometry.
It is a known fact that very good noise reduction values are achieved with a stepped and a so-called Laval nozzle hole geometry (Nakano, Tajima, Kumaido: A Study of Improvement of Noise Suppression in a Low-Noise Valve of the Multi-hole Type, 2nd report, Nippon Kikai Gakkai Robunshu, B Hen/Transactions of the Japan Society of Mechanical Engineers, Part B, Vol. 55, No. 520; Seiner: Acoustic Near-Field Properties Associated with Broadband Shock Noise, AIAA Journal, Vol. 22, No. 9; Tam: On Broadband Shock Associated Noise of Supersonic Jets, Recent Advances in Aeroacoustics, Proceedings of an International Symposium, Springer Verlag, NY. 1986).
A stepped nozzle means a nozzle where the point of connection between the throttling part and the following expanded part is formed by a plane which is perpendicular to the flow direction or by a relatively short expanding cone. A Laval nozzle is one where the initial channel area is first reduced and tapering and then again increases and becomes conical or curved, finally discharging into a wide space. The major problem in noise reduction with a Laval nozzle is that good reduction capability is obtained only within a very narrow pressure drop ratio range. In practice, this may mean an increase of up to 20 dB in the noise level, with only a small change in the noise ratio. Such a change can mean that the pain threshold is exceeded. However, in control valves, for example, the pressure drop ratio may vary even over a wide range depending on the opening position of the control means and on the process to be controlled.